Radio resource management techniques are employed to utilise the limited radio spectrum resources and radio network infrastructure as efficiently as possible. Modern cellular communications systems such as wideband code division multiple access (WCDMA) systems implement a plurality of such techniques.
FIG. 1 shows an illustration of an exemplary WCDMA system architecture 10 including a core network 12, a radio access network (RAN) 14 and a plurality of user terminals 16, also referred to as user equipment (UE). The RAN includes one or more components 18 responsible for radio network control (RNC) and one or more base station components 20, also referred to as “Node B”, that mainly perform air interface processing. Each base station component 20 serves one or more network cells. One RNC component 18 and one or more associated base station components 20 constitute a radio network subsystem (RNS). A RAN typically comprises a plurality of such RNSs.
Enhancements in the uplink direction of WCDMA are currently being standardised within the 3rd generation partnership project (3GPP). Among the various standardized features are fast scheduling and fast hybrid automatic repeat request (HARQ) as described in the 3GPP document TS 25.309 “FDD Enhanced Uplink”. Conventional radio resource management techniques include control features such as admission and congestion control (ACC), radio link control (RLC), and outer loop power control (OLPC). As shown in FIG. 1, these features are conventionally located in the RNC component 18. On the other hand, the new control features introduced for enhancing the uplink direction, such as fast uplink scheduling and fast HARQ, are primarily located in the base station components 20.
The document TS 25.309 not only describes new control features, but also new uplink channels. In addition to conventional uplink channels such as the dedicated physical data channel (DPDCH) and the (high speed) dedicated physical control channel ((HS-) DPCCH), an enhanced DPDCH (E-DPDCH) and an enhanced DPCCH (E-DPCCH) are introduced. The DPCCH carries pilot symbols and portions of the outband control signalling. Remaining outband control signalling for implementing the enhancements in the uplink direction is carried on the E-DPCCH, while the E-DPDCH carries the data transmitted using the enhanced uplink features. According to TS 25.309, the term E-DCH generally denotes a new dedicated transport channel type or enhancements to an existing dedicated transport channel type.
In the following, the radio resource management technique of fast uplink scheduling will be discussed in more detail. Generally, fast scheduling as used in the uplink context here denotes the possibility for a base station component 20 to control when a user terminal 16 is transmitting and, in combination with adaptive modulation and coding (AMC), at which data rate.
Using the fast scheduling feature, the base station component 20 sends a resource indication (“scheduling grant”) in the downlink to the user terminal 16. The scheduling grant indicates to the user terminal the maximum amount of uplink resources the user terminal is allowed to use. The scheduling grants are used in connection with the E-DCH transport format combination (TFC) selection and control the maximum allowed E-DPDCH/DPCCH power ratio. In general, the scheduling grants set an upper limit on the data rate a particular user terminal may use. However, the power situation in a particular user terminal, as well as activity on other, non-scheduled channels, may lead to the situation that the user terminal transmits with a lower data rate on the E-DCH than that granted by means of the scheduling grants.
The scheduling grants can be divided into absolute grants on the one hand and into relative grants on the other. By using these two types of grants, the scheduling base station components can sophistically control the transmission behaviour of each individual user terminal.
Absolute grants are used to set an absolute limitation (typically in terms of power ratio relative DPCCH) for the maximum amount of uplink resources that may be used on the E-DCH for data transmission. The maximum amount of uplink resources allowed for E-DCH data transmission determines the maximum data rate on E-DCH. Typically, absolute grants are used for significant but infrequent changes of the resource allocation for a particular user terminal (e.g., at times of bearer setup or when granting resources in response to a scheduling request received from a user terminal).
Absolute grants are sent by the E-DCH cell serving a particular user terminal and transmitted on a control channel called E-AGCH (E-DCH absolute grant channel) that can be shared by multiple user terminals. Generally, there is only a single E-AGCH for all user terminals that are served by a particular cell.
Relative grants, on the other hand, are used to update the resource allocation for a particular terminal. Relative grants can be sent by serving as well as non-serving base station components and typically as a complement to absolute grants. A relative grant from a serving cell can take one of three different signalling contents, namely either “up”, “down” or “hold”. A relative grant from a non-serving cell can take one of two different values, “down” or “hold”. These signalling contents refer to uplink resource limitations associated with a user terminal relative to the amount of resource the user terminal is currently using.
Relative grants are transmitted on individual control channels, namely on E-DCH relative grant channels (E-RGCHs). FIG. 3 shows a schematic illustration of E-RGCH and E-AGCH signalling. There is one E-RGCH per user terminal from the serving cell, and each user terminal may receive one relative grant per transmission time interval (TTI). Thus, the relative grants have some similarities with power control instructions.
In a soft handover scenario, in which a user terminal is communicating with a plurality of cells, the user terminal receives absolute grants only from a single one of these cells, namely from the serving E-DCH cell (or simply serving cell). The serving cell has therefore the main responsibility for the scheduling operation. However, also non-serving cells involved in a soft handover with a particular user terminal are able to influence the resource consumption of this user terminal in order to control the overall interference level within their own cell coverage. In this context, a particular user terminal may receive relative grants from both the serving cells and all non-serving cells involved in a soft handover with the particular user terminal.
A serving E-DCH radio link set (or simply serving RLS) denotes the set of cells which contains at least the serving cell and from which the user terminal can receive relative grants and absolute grants. Each user terminal has only one serving RLS. A non-serving E-DCH RLS (or simply non-serving RLS) denotes the set of cells which does not contain the serving cell and from which the user terminal can receive absolute grants. A user terminal may have zero, one or several non-serving RLSs.
Base station components of the non-serving RLS will only send relative grants to the user terminal. The relative grants from such base station components are restricted to the values “down” and “hold”. In the absence of a “down” from any non-serving RLS, the user terminal simply follows the scheduling grants of the serving RLS.
If a user terminal is receiving a “down” from any non-serving cell, this is an indication that the cell in question is overloaded and the user terminal shall therefore reduce its data rate compared to the data rate it is currently using (even if one or more grants from the serving cell suggest an increase). Thus, the relative grant from a non-serving cell serves as an overload indicator. The overload indicator is sent to all user terminals for which the overloaded cell is a non-serving cell as shown FIG. 3.
The challenge on the side of the user terminals in context with decoding the (serving or non-serving) E-RGCH is that the relative grants are multi-valued (“up”, “down”, “hold”), and that most of the time a “hold” will be signalled. Signalling a “hold” basically means that the transmitted amplitude level on the E-RGCH is zero.
A possible approach for decoding the E-RGCH might be to determine a threshold τE-RGCH>0 and to decide for “down” if γE-RGCH<−τE-RGCH (with γE-RGcH being the demodulated sample value for the E-RGCH), “up” if γE-RGCH>τE-RGCH, and “hold” otherwise.
τE-RGCH may be fixedly determined based on a noise variance estimate and may be selected to realize a fixed probability of, for example, 0.1 for missing a “hold”. Accordingly, the probability of missing either a “up” or a “down” is 0.05 in each case. In situations of good channel conditions, for example in cases of increased E-RGCH power, it would be possible to have both a lower probability for a missed “hold” and a lower probability for a missed “up”/“down”. However, such situations are difficult to exploit because the E-RGCH power offset relative to, for example, the common pilot channel (CPICH) is not signalled. Moreover, it is expected that the base station components will signal “hold” most of the time, which makes it difficult to estimate the power on the E-RGCH in a simple manner.